1. Field of the Invention
The present invention relates to an IC for control of a temperature-compensated crystal oscillator (TCXO), and more particularly, to a crystal oscillator and common use of terminals of an IC for control thereof.
2. Description of the Related Art
Modern crystal oscillators have been rapidly downsized. In addition, semiconductors for controlling crystals have been rapidly downsized. A crystal oscillator typically has a power terminal (VCC1), a ground terminal (GND), a frequency output terminal (OUT) and a frequency control terminal (VC) (having no frequency control function in some cases). In addition, the crystal oscillator requires connection terminals (XT1, XT2) of a crystal vibrator.
In addition, a crystal oscillator performing oscillation with high precision contains a PROM circuit used to control a temperature-compensated circuit and so on (TCXO, VCXO, etc.).
TCXO performs high-precise oscillation through control to cancel frequency-temperature characteristics of crystals. Since different crystals have different frequency ununiformities, an adjustment is made to each of TCXOs, and the TCXOs perform temperature compensation based on adjustment data written in a PROM circuit contained in a control IC. Hereinafter, frequency adjustment of a TCXO using a ONE-time PROM will be described.
Existing frequency adjustment of TCXO requires three kinds of operation modes. Prior to use in mode typically used, in order to determine data to be written in a PROM circuit (write bits), there is a mode for inputting the data and acquiring and calculating frequency data (hereinafter referred to as an emulation mode (EM)). In addition, there is a mode for writing the data determined in the emulation mode (EM) in the PROM circuit (hereinafter referred to as a write mode (WM)). Then, in a mode typically used (hereinafter referred to as a read mode (RM)), the data written in the PROM mode is read out and frequency of the data is controlled. By switching between these three kinds of modes, frequency adjustment of TCXO is made.
A crystal oscillator performing oscillation with high precision in this manner requires dedicated control terminals for control in the above modes. For example, a CLK terminal, a DATA terminal and a PROM terminal are required for control of the PROM circuit. However, there is a need of common use of these terminals to further miniaturize the crystal oscillator.
As an example of the terminal common use, Patent Document 1 discloses a technique related to common use of a frequency control terminal and a DATA terminal and common use of an oscillation output terminal and a CLK terminal.
The present applicant has already suggested a TCXO module having a terminal (Vcc, CLK) 56 used for both of a power terminal and a CLK terminal and a terminal (DATA/VC) 52 used for both of a DATA terminal and a VC terminal (see Patent Document 2), as shown in FIG. 11. The TCXO module shown in FIG. 11 has 8 terminals in total, additionally including connection terminals XT 51 and XTB 55 of a crystal vibrator, a PROM voltage terminal 53, a write PE terminal 57, a ground terminal (GND) 54 and an oscillation output terminal (OUT) 58.
FIG. 23 is a schematic view showing a configuration of the temperature-compensated crystal oscillator disclosed in Patent Document 2. In FIG. 23, a power/CLK terminal 11A is a module power terminal through which a signal voltage of 2.8 V is supplied. In addition, this terminal 11A also serves as a CLK terminal through which a clock signal is delivered to a PROM circuit. A voltage control/DATA terminal 11A is a terminal for controlling a frequency with a voltage and serves to deliver data to the PROM circuit.
In this manner, this temperature-compensated crystal oscillator commonly uses the CLK terminal and the power terminal through which the clock signal is delivered to the PROM circuit, and commonly uses the voltage control terminal through which a voltage is applied to a voltage control circuit and the DATA terminal through which data are delivered to the PROM circuit.
FIG. 24 is a view showing adjustment data input waveforms in the temperature-compensated crystal oscillator. In adjustment, after a power terminal 5A is set to 2.3 V, a clock signal is delivered to a register of a control IC through the power terminal 11A and a data signal is delivered to the register through the terminal 12A. After the delivery of these signals, the control IC is fixedly set to a power voltage of 2.3 to 3.3 V used typically. In addition, a voltage of 0 V to 3.3 V in a voltage control range is applied to the terminal 12A for oscillating frequency adjustment.
If a memory is configured with a rewritable EEPROM, there may be provided, in some cases, no emulation mode if there is little margin for a chip area. In this case, since frequency data are taken, write is purposely performed and data are taken on every occasion. Thereafter, optimal bits are calculated based on the data. Subsequent operations are the same for any PROMs.
In this case, although a basic adjustment method is not changed, details of a method of taking data in emulation are as follows. Data of emulation are actually inputted. A frequency is no changed at this point of time. Next, write operation is performed. This allows data to be reflected on an output. Data are taken with repetition of this operation.
Patent Document 1: U.S. Pat. No. 5,724,009
Patent Document 2: JP-A-2003-188646
However, since the method disclosed in Patent Document 1 uses terminals in common with an analog signal and a digital signal, very complicate switching is required to adjust the crystal oscillator.
For example, for switching in common use of the OUT terminal and the CLK terminal, while a pulse generating power source is connected in CLK input, since a frequency counter for reading a frequency from the OUT terminal is required in a normal state, switching of connection is complicate.
In the meantime, when the PROM circuit is used to adjust the crystal oscillator, there is a need to input serial data in order to set the emulation mode (EM), thereby requiring the CLK terminal and the DATA terminal. In addition, terminals for switching between the read mode (RM), the emulation mode (EM) and the write mode (WM) are needed. The following description shows specific examples.
FIG. 12 shows a conventional TCXO control IC. The conventional TCXO control IC has power terminal 2 (VCC2: power for mode select) 62 in which mode switching is performed. For example, modes are distinguished from each other in such a manner that the read mode (RM) is used when the power terminal 2 (62) is open, the emulation mode (EM) is used when the power terminal 2 (62) is applied with 2.3 V, and the write mode (WM) is used when the power terminal 2 (62) is applied with 3.5 V.
In this manner, in switching between the read mode (RM), the emulation mode (EM) and the write mode (WM), the conventional TCXO control IC performs data input by providing a bias from the power terminal 2 (62) to a power terminal (PROM power VCC2: power for mode select). However, there is a need to further reduce PAD (terminal) of IC or terminals of a TCXO module to meet a demand for recent miniaturization.